This invention relates to cascade control systems. In the control of some processes it is possible to utilize an intermediate process variable that responds to both the manipulated variable and to some disturbances to provide a secondary control to achieve more effective control over the primary process variable. This technique is called "cascade control". Cascade control is essentially the use of the output of a primary controller to adjust the set point of a secondary controller.
In utilizing cascade control, frequently the dynamics of the secondary process which produces the intermediate variable are not considerably faster than those of the primary process, so that the advantages of cascade control are reduced in that the intermediate variable, which will hereafter be called the secondary variable, cannot be corrected by the secondary controller before a pronounced influence is felt by the primary variable. Also, the advantages are reduced if the secondary controller must have a low gain and/or a high reset rate. In some systems attempts to tune both the primary and secondary controllers to give good response to the disturbances affecting both the primary and secondary processes may lead to oscillatory or unstable control. Some attempts have been made to avoid such results. For example, the system shown in FIG. 7.8e on page 824 of Volume II of Instrument Engineers Handbook by B. G. Liptack, published by Chilton Book Company, New York, shows a system wherein the secondary controlled variable is utilized not only as an input to the secondary controller but is also utilized as a reset input to the primary controller. Systems of this type will not provide the necessary decoupling of the primary and secondary control systems to avoid interaction as is desired.
It is an object of this invention to improve upon the cascade control systems used heretofore to overcome the interactions between the primary and secondary controllers by a novel method of decoupling.